Unite and GMB are holding strike ballots over a “completely unacceptable” pay increase offer that was rejected by union members at Sellafield nuclear facility.

Of the site’s 11,000 staff members, 5,000 belong to either Unite or GMB.

The majority of Sellafield staff, which belong to the Prospect union, have accepted the pay offer, Sellafield said in a statement.

“We’ve offered our workforce a 1.5% increase this year – which is an increase to our wage bill of over £12 million year-on-year,” the statement said.

It added: “After a ballot of their members this has been accepted by one union, which has collective bargaining rights for over half the workforce, and rejected by two others who, between them, have collective bargaining rights for the remainder.”